1. Field of the Invention
This invention relates to a rocker arm formed by pressing which is provided in the valve opening and closing mechanism of an engine.
2. Discussion of the Related Art
In an engine, a rocker arm operates as follows: As the cam is rotated in association with the crank shaft, the rocker arm is swung about its one end portion on the pivot side, while causing its other end portion to push the valve stem. In an example of the rocker arm, its body is formed by pressing.
An example of the rocker arm formed by pressing is as shown in FIGS. 4 and 5. In FIGS. 4 and 5, reference character C designates a cam; V, a valve; and P, a lash adjuster's pivot. The rocker arm comprises a rocker arm body 1', and a roller 2' brought into contact with the cam C.
The rocker arm body 1' is so formed by pressing a metal plate that it is U-shaped in section having side walls 3' and 3' on its both sides, and a bottom between those side walls. The bottom has a rectangular through-hole 4' substantially at the middle as viewed longitudinally thereof. The roller 2' is located in the through-hole 4', and rotatably supported on the side walls 3' and 3' through a pin 5'. More specifically, one end portion (the right end portion in FIG. 4) of the bottom of the rocker arm body 1' is formed into a semi-spherical portion, namely, a pivot engaging portion 6' which is abutted against the top of the pivot P, whereas the other end portion (the left end portion in FIG. 4) is formed into a valve engaging portion 7' which is abutted against the top of the valve stem V.
On the other hand, there is a general demand for reduction of the width of the rocker arm in the art. This requirement cannot be satisfied by the conventional rocker arm formed by pressing, because, although its width on the valve side can be reduced, it is difficult to reduce its width on the pivot side.
As was describe above, in the conventional rocker arm formed by pressing (hereinafter referred to as "a conventional rocker arm", when applicable), one end portion of the bottom of the rocker arm body 1' U-shaped in section is formed into the pivot engaging portion 6'. In order to provide the pivot engaging portion 6' with high accuracy, it is essential that the periphery of the pivot engaging portion 6' is flat. Hence, there must be a certain distance D between the pivot engaging portion 6' and each of the side walls 6'; otherwise the side walls adversely affects the configuration of the pivot engaging portion 6'.
The diameter of the pivot P of the lash adjuster has been predetermined, and accordingly the diameter of the pivot engaging portion 6' also has been predetermined, which causes the distance between the end portions of the side walls which are located beside the pivot engaging portion 6' (hereinafter referred to as "pivot-side portions", when applicable) to be larger than a certain value. That is, in each of the side walls 3', its middle portion 31' is shifted from its pivot-side portion 32' as shown in FIG. 5.
This fact adversely affects the manufacture of the conventional rocker arm. That is, in the conventional rocker arm, the roller supporting pin 5' is inserted into the side walls 31' of the rocker arm body 1', which is V-shaped in section and secured to it by caulking. However, it may be rather difficult to positively secure the pin 5' to the side walls by caulking, because the fixing of the rocker arm body 1' during the caulking process may be loosened or unstable due to the above-described structure that the middle portions 31' of the side walls are shifted from the pivot-side portions 32' of the latter.